The invention relates to video systems, and relates specifically to systems for forming a composite signal by combining digital information with an analog video signal and for recovering the digital information from the composite signal.
A number of systems exist for transmitting information in conjunction with an analog video signal. In order to avoid flicker or spotting at the TV receiver, a number of these prior art systems modulate portions of the transmitted signal which are outside the visible scanlines. Typically, systems of this type modulate scanline sync pulses, transmit control information during the vertical blanking interval of a frame field, transmit control information between video cuts, or utilize a separate frequency band for transmitting additional information. Systems of this type are disclosed in U.S. Pat. Nos. 3,752,907; 3,493,674; 3,730,986; and 3,145,262. Since these systems transmit information outside the visible scanlines of a video signal, they are capable of utilizing only a small portion of the total video transmission time for conveying additional information, and are therefore likely to have a slower rate of transmission and possibly a higher occurrence of errors than a system capable of transmitting information concurrently with the visible portion of the video signal.
One system for superimposing digital information on the visible portion of an analog video signal is shown in U.S. Pat. No. 3,061,669 to Leek. In Leek, the output of a first TV camera is converted to a pulse-amplitude-modulated train of narrow, high-amplitude pulses which are then superimposed on the visible scanlines of a second TV camera output. At the receiver these narrow pulses are removed from the analog video information and are converted back to an analog video signal by amplitude demodulation and pulse stretching. While the Leek disclosure is very sketchy and does not discuss this question, it would appear that the Leek system would require additional bandwidth for transmission of the composite signal. Additionally, since there is nothing to prevent the occurrence of the same pattern of narrow pulses on successive scanlines or frames of the analog video signal, it would appear that visible flicker and spotting on the video receiver could not be prevented. A similar technique applied to an audio signal is shown in Tomsa, U.S. Pat. No. 3,735,048. In Tomsa, an audio signal is interrupted by short pulses conveying additional information. Since these pulses can occur at any time, this system is not suitable for use in video applications where visible flicker and spotting may result, and the pulses may even interfere with the horizontal and vertical sync signals.
Other known techniques for transmitting additional information in conjunction with a video signal include inversion of selected scanlines, and transmission of information in a narrow portion of at least one scanline per frame, as shown in U.S. Pat. Nos. 3,069,492 and 3,746,780, respectively. A technique for introducing additional digital data into a digitized signal stream is shown in U.S. Pat. No. 3,727,005.
The prior art discussed above indicates that numerous systems have been developed to transmit information on portions of a video signal that are outside the visible scanlines. When digital information is applied to the visible portion of video scanlines, as in Leek, no provisions are made to prevent the occurrence of repetitive pulse patterns on successive scanlines, which are likely to cause visible flicker and spotting on the received image. Furthermore, in the prior art discussed above no provision is made for maintaining the fidelity of the video image while concurrently minimizing the bandwidth of the composite signal.